An immunoassay utilizes antibodies to detect a compound of choice. However, the sensitivity of this detection is generally limited to the amount of signal that can be carried either on an antibody, for a direct binding assay, or on the probe compound, in a competitive inhibition assay. For example, in existing immunoassays, such as radioimmunoassays, ELISA, immunofluorescent assays or immunochemiluminescent assays, too many signal entities, such as radioisotopes, horse radish peroxidase or alkaline phosphatase, attached to the detection moieties invariably inactivate the antibody or denature the antigen and change the property of he detection probe. Therefore, in order to obtain more signal, additional antibody or probe must be added. This, in turn, reduces the sensitivity of the assay, the capability of the assay to detect minute quantities of the compound in question.
For all existing immunoassays, there is lag time for the compound of interest to reach a high enough concentration in the serum to become detectable for diagnostic purposes. In the case of heart attacks, there is a delay of 4-6 hours from the onset of chest pain until the diagnostic detection of CK-MB, Troponin-T or I is possible. Myoglobin is detectable earlier, but its specificity is low. If there were an assay that could detect very minute increases of these indicator compounds in the blood at an earlier point in time, then therapeutic intervention could be started earlier and thereby bring about greater myocardial salvage. In the case of cancer detection, where, e.g., tumor associated antigens related to breast cancer or colon cancer, etc., are detected, treatment might be more effective if minute elevations of these antigens could be detected at an early stage. Therefore, there is a need to increase the sensitivity of the assay without adversely affecting the specificity of the assay system.
The invention is directed to a method to increase the sensitivity of an immunoassay, by at least 10,000 fold, without losing specificity. This improvement is achieved by the use of a bispecific antibody complex and a unique detection signal probe capable of recognizing the bispecific antibody complex.
In one aspect, the invention features an immunoassay method including reacting a sample from a patient with a bispecific antibody, wherein the bispecific antibody includes one antibody specific for a compound to be detected and a second antibody specific for a compound foreign to said patient sample, i.e., non-endogenous, and subsequently reacting the patient sample with a polymer probe, wherein the polymer probe includes a compound recognized by the second antibody in the bispecific antibody complex and further includes at least two detectable signals. The invention also features the bispecific antibody and the polymer probe of the method of the invention. Preferably, the sample from the patient is a blood or serum sample; the bispecific antibody includes an antimyosin antibody and an antibody against DTPA; and the polymer probe is a polylysine polymer and includes DTPA and at least six HRP as the detectable signal compounds.